The present invention relates to a magnetic detector using magnetoresistive elements. The magnetic detector can be applied to magnetically detect the angle, position or speed of rotation of a rotating body or the rectinear position, distance or speed of a rectinearly moving body. The applications include the detection of the angle or number of rotation(s) of an electric motor and the control of the rotational speed based thereon, the detection of the quantity of feeding of the working arm or cutting tool of an automatic machine such as a robot and the control of the feed speed based thereon, or the detection of the quantities of various feedings in business machines such as a copier or printer and the control of the feed quantities based thereon.
When it is desired to determine the rotational position of a magnetic recording medium, for example, a magnetic detector (hereinafter referred to as magnetic head) is provided with a magnetoresistive element arranged opposite to the magnetic recording medium which moves relative thereto. Such a magnetic head is disclosed in, for example, Japanese Patent Application Laid-Open No. 87862/81 entitled "Magnetic Head for Rotation Detector".
FIG. 1A shows the arrangement pattern of magnetoresistive elements disclosed in the Japanese Patent Application Laid-Open No. 87862/81. In the figure, reference numeral 3A-1 designates a magnetic recording medium and numeral 4A generally designates a magnetic head. Reference numeral 5A designates a magnetoresistive element pattern in the magnetic head, numeral 6A a non-magnetic substrate, symbols a to d terminals for external connection, and symbols R.sub.11, R.sub.12, R.sub.21, R.sub.22, R.sub.31, R.sub.32, R.sub.41 and R.sub.42 magnetoresistive elements formed on the terminals. The magnetic recording medium 3A-1 shown in FIG. 1A is an enlarged version of a portion of a drum-like magnetic recording medium shown in FIG. 3 which will be later described. FIG. 1B shows a bridge circuit for the magnetoresistive element pattern 5A.
As shown in FIG. 1A, magnetic poles N and S are recorded in the magnetic recording medium 3A-1 at regular intervals, and the magnetoresistive element pattern 5A includes the terminals a to d for external connection. The magnetoresistive elements are arranged between the terminals a to d so as to form the bridge circuit shown in FIG. 1B.
Ones of the magnetoresistive elements making up an arm of the bridge circuit, for example, the elements R.sub.11 and R.sub.12 are combined in U-shaped hairpin configuration. Therefore, when the recording pitch of the magnetic poles N or S is expressed by .lambda., it is required that the total width of all the magnetoresistive elements forming the bridge circuit is (2+3/4).lambda. as understood from FIG. 1A.
In many rotation detectors, it is also required to determine the direction of rotation, and therefore two outputs having a phase difference therebetween are provided. In this case, a pair of bridge circuits are required and hence the total width of magnetoresistive elements will be more than twice larger than (2+3/4).lambda..
FIGS. 2 and 3 illustrate how the magnetic head having the above-described magnetoresistive elements is combined with a magnetic recording medium which is to be detected by the magnetic head. In these figures, reference numeral 1 designates a motor, numeral 2 a shaft of the motor, numeral 3A a disc-like magnetic recording medium, numeral 3A-1 a drum-like magnetic recording medium, and numerals 4A and 4A-1 magnetic heads. In the arrangement shown in FIG. 2, the magnetic head 4A is disposed opposite to one side surface of the disc-like magnetic recording medium 3A. In the arrangement shown in FIG. 3, the magnetic head 4A-1 is provided opposite to the outer peripheral surface of the drum-like magnetic recording medium 3A-1 extending in the derection of drum axis. One of these arrangements is selected in accordance with demand.
The amplitude of a detection signal output derived from a magnetoresistive element corresponds to only a 2 to 3% in terms of the change in resistance thereof. Moreover, this change in resistance greatly depends upon the distance between the magnetoresistive element and the magnetic recording medium (hereinafter referred to as "spacing"), as disclosed in Japanese Patent Application Laid-Open No. 90213/81. FIG. 4 shows a relation between the spacing and the output amplitude of a magnetoresistive element. As is apparent from the figure, the output amplitude exhibits remarkable decrease as the spacing becomes larger. In a rotation detector using a magnetoresistive element with such a characteristic as a detecting element, therefore, it is important that the spacing does not change so that the output amplitude is kept constant.
In order to obtain stable spacing, the arrangement shown in FIG. 3, is more advantageous than that shown in FIG. 2 since a machining operation for suppressing the roughness of the outer peripheral surface of a drum in the drum axis direction can be performed with higher accuracy as compared with that for suppressing the roughness of the side surface of a disc and since when the magnetic recording medium is incorporated into a machine a variation in spacing due to any load in the drum axis direction is negligible.
In the arrangement shown in FIG. 3, however, there is a problem that when the total width W of magnetoresistive elements is large, a substantial difference exists between the spacing S.sub.1 for a magnetoresistive element located at the central portion of a magnetic head and the spacing S.sub.2 for a magnetoresistive element located at the peripheral portion of the magnetic head, as shown in FIG. 5. Namely, though the central magnetoresistive elements give large output amplitudes, the outer magnetoresistive elements will give small output amplitudes, thereby resulting in an insufficient bridge circuit output or an unusual distortion of the detection signal waveform.
Thus, when the combination of magnetoresistive elements in hairpin configuration as shown in FIG. 1A are used in the arrangement of FIG. 3, the above-mentioned inconvenience arises due to the large total width of magnetoresistive elements. The same inconvenience will arise even in the arrangement of FIG. 2 when the spacing between the magnetic head and the disc-like magnetic recording medium is not uniform.